A subject matter of the present invention is a process of use in the preparation of capsules comprising inorganic shells of controlled thickness and having a core composed of an aqueous fluid in which at least one active material can be dispersed and/or dissolved.
Various microencapsulation techniques are already available for packaging varied active materials. This type of packaging is favored in particular when it is desired to mask the taste of an active material, to control its release over time and/or to protect it from its surrounding medium.
Overall, two types of encapsulation techniques can be distinguished.
The first type results in a xe2x80x9creservoirxe2x80x9d system. The active material is immobilized at the center of a capsule by a membrane of polymeric nature. Capsules of this type are generally obtained by the technique referred to as the interfacial polycondensation technique. It consists in bringing about the condensation of a polymer at the interface of two immiscible liquids. Microspheres with a mean diameter of 10 to 30 xcexcm composed of a liquid core surrounded by a thin polymer envelope representing only 5 to 15% of the total weight of the capsule are thus obtained. The polymer membrane provides for protection with respect to the external medium of the liquid core, which generally comprises an active material, and its porosity makes it possible to control the diffusion thereof out of the capsule. This type of encapsulation is provided in particular for the packaging of active materials of pesticidal type for which it is desired to control the release over time. Techniques referred to as coating by spraying, coating by phase separation and coating by solidification also result in this xe2x80x9creservoirxe2x80x9d system.
The second type of encapsulation technique results in a xe2x80x9cmatrixxe2x80x9d system. The active material being encapsulated is dispersed within either an organic network of polymer type or an inorganic network, such as, for example, silica. Mention may more particularly be made, by way of representation of this type of encapsulation, of the technique of inorganic encapsulation by the sol-gel route. The convention sol-gel technique consists in initiating the hydrolysis and the polycondensation of a metal alkoxide in an aqueous or aqueous/alcoholic medium comprising the active material to be packaged. This results in the formation of a gel in which said active material is found dispersed, which gel leads, after drying, to a porous glass. According to an alternative form of the sol-gel technique, referred to as the emulsion sol-gel technique, a network of inorganic oxide, generally of silica, is formed from a molecular precursor of the alkoxide type in the presence of a water-in-oil emulsion in which the active material is dispersed. The active material present during the stage of hydrolysis and the condensation of the inorganic material is then trapped in the powder.
The present invention is targeted more specifically at providing a novel encapsulation technique which makes it possible to produce capsules comprising a core composed of an aqueous fluid surrounded by an inorganic shell.
More specifically, a subject matter of the present invention is a process for preparing inorganic capsules composed of an aqueous liquid core surrounded by an inorganic shell, said process comprising:
1) emulsifying an aqueous fluid within an organosoluble phase which is immiscible with said aqueous fluid, so as to disperse it therein in the form of droplets,
2) bringing into contact, within the emulsion thus obtained, at least one hydrolyzable and polycondensable compound of zirconium, of silicon, of aluminum and/or of a transition metal under temperature and pH conditions favorable to the formation of an inorganic precipitate composed of the corresponding oxide or hydroxide, and
3) recovering the inorganic capsules thus formed and, if appropriate, purifying them, said process being characterized in that the formation of the inorganic precipitate in stage 2 is carried out in the presence of an amphiphilic surfactant system present in the emulsion and capable of concentrating the deposition of the inorganic particles of the precipitate formed at the interface of the aqueous droplets and of the organosoluble phase and of effectively blocking the diffusion of these inorganic particles within said droplets.
In the context of the present invention, it is possible to envisage packaging simply an aqueous fluid, such as water. According to this alternative form, the water is used directly as active material. It can thus represent an agent for delayed hydrolysis of chemical compounds, such as functionalized silicones.